1. Field of the Invention
The invention relates generally to a transition metal complex and more particularly to a transition metal complex used in an electronic device.
2. Background Information
Research and development relating to various kinds of display devices are actively carried out today. In particular, organic electroluminescent elements have received attention as promising display elements because of their ability to emit light at a low voltage. For example, a light-emitting device having an organic thin film formed by vapor deposition of an organic compound has been known (Applied Physics Letters, Vol. 51, p. 913 (1987)). Research has revealed that high power efficiency organic electroluminescent devices, such as organic light emitting diode (OLED) devices, can be made using molecules that emit light from their triplet state.
Light emission from the triplet state, as opposed to light emission from the singlet state, is referred to as phosphorescence. The triplet excitons can produce phosphorescence, whereas the singlet excitons typically produce fluorescence. Since the lowest emissive singlet excited state of an organic molecule is typically at a slightly higher energy than the lowest triplet excited state, the singlet excited state may relax, by an intersystem crossing process, to the emissive triplet excited state. This means that all the exciton excitation energy may be converted into triplet state excitation energy, which then becomes available as phosphorescent emission. Since all the exciton excitation energy can become available as electrophosphorescence, electrophosphorescent devices have a maximum theoretical quantum efficiency of 100%.
In recent years, much effort has been geared to application of organic electroluminescent devices to full color displays and white light sources. As known, full color display devices require red, green, and blue light sources (RGB). Thus, in order to produce a full color display that takes advantage of the high power efficiency of organic electroluminescent devices, a triplet emissive blue is needed as well as high efficiency triplet emissive red and green materials. Examples of triplet emissive red and green materials are already in existence. However, triplet emissive blue has been difficult to attain due to the high energy of the emissive state. Efficient long-lived blue-light emitters with good color coordinates are a recognized current shortfall in the field of organic electroluminescent devices.
Although some light-emitting devices have been reported to emit blue light, there has been no report of blue light-emitting device having an external quantum efficiency exceeding 5%. If a blue light-emitting device having an external quantum efficiency exceeding 5% could be provided, application of light-emitting devices would be greatly expanded. Moreover, it would be possible to significantly reduce electrical power consumption, since electrical appliances use these electroluminescent devices.